For example, Eric saying "peekaboo" and then shooting Cassie Bernall in the face with a shotgun.
I can understand being so ruthless if it was the jocks who had been making their lives hell... but Cassie and some of the other brutal deaths were people who AFAIK they didn't even have a problem with.
I guess one theory is that the boys were disassociated during the shooting and felt like they were in a giant videogame or movie, and that in their right minds they wouldn't be capable of such utter disregard for human life.
It's also possible that the boys considered every student of Columbine as "people who looked the other way" and therefore undeserving of mercy.
Your thoughts?